[unreadable] Yersinia pestis is the causative agent of bubonic, pneumonic and septicemic plague, and is currently a major concern as a potential bioterrorism weapon. An attack in the United States with aerosolized Y. pestis, which induces the fatal pneumonic form of the disease, would have severe consequences on human lives and the economy. Sporadic outbreaks of the plague also continue to occur naturally in Africa, Asia and the Americas. Strains of Y. pestis that are resistant to multiple antibiotics have recently been isolated, suggesting that natural outbreaks of plague may become more difficult to control. Currently, there are no effective vaccines to protect against Y. pestis, and unless antibiotics are administered immediately after infection, the disease is usually fatal. Research to identify new strategies to control Y. pestis will require genetic manipulation of the pathogen. However, most techniques in current use require the introduction of genetic elements that carry antibiotic resistance markers. Antibiotic resistant Y. pestis poses a health concern not only to researchers, but also as an enhanced risk if resistant strains are obtained by bioterrorists. This research seeks to test the main hypothesis that mutant repressor proteins will function as selectable markers by imparting resistance to potent metabolic inhibitors. These studies will provide new tools for genetic manipulation of Y. pestis that will lead to a better understand its pathogenicity as well as to the production of effective vaccines, without increasing virulence. The methods described here will also be applicable to other select agents of bioterrorism. [unreadable] [unreadable] [unreadable]